1. Field of the Invention
The present invention relates to a heating device utilizing a positive temperature coefficient (PTC) ceramic resistor in which an electric resistance value varies greatly by the order of about three to seven figures at and around a Curie temperature. More particularly, it relates to a self-temperature controlling type heating device which has a self-temperature temperature controlling function and in which the temperature to be controlled is variable.
2. Description Of The Prior Art
A positive temperature coefficient ceramic resistor has heretofore been utilized widely as an electric material; for example, it has been used practically as a contactless current control device for a motor start switch, as a temperature compensating thermistor and as a self-temperature controlling type heating device in a hair drier, a warm air heater, etc.
The specific resistance of a positive temperature coefficient ceramic resistor consisting principally of barium titanate (BaTiO.sub.3) has heretofore been about 5 ohm.multidot.cm as a minimum value. Therefore, a self-temperature controlling type heating device using such a positive temperature coefficient ceramic resistor in the form of a thick film has encountered a limit in obtaining a high output at a low voltage. If the thickness of the positive temperature coefficient ceramic resistor is made smaller in order to compensate for this drawback, the positive temperature coefficient will be lowered, making it difficult to effect temperature control. Further, since the heat generating temperature of a positive temperature coefficient ceramic resistor is determined directly by its Curie temperature and the amount of radiation heat, it has been difficult to control the temperature in use.
Having made extensive studies about the positive temperature coefficient of a ceramic resistor, the present inventors found that the said coefficient was caused by grain boundaries of a positive temperature coefficient ceramic resistor. More particularly, the following phenomenon was found out. With increase of grain boundaries of a positive temperature coefficient ceramic resistor, the resistance variation width increases and the positive temperature coefficient appears remarkably. On the other hand, if a positive temperature coefficient ceramic resistor is constituted by a single layer of crystals to eliminate a grain boundary, the positive temperature coefficient disappears.